It is known in the art to directly bond two semiconductor wafers, such as two silicon semiconductor wafers. For example, a P type wafer and an N type wafer can be bonded together to form a P-N junction. Direct bonding of wafers is disclosed in U.S. Pat. No. 4,939,101 dated Jul. 3, 1990, and assigned to the assignee of the present application, the disclosure thereof being hereby incorporated by reference.
Wafer bonding is commonly achieved by sintering together the "bare" surfaces of two individual semiconductor substrates, such as two silicon wafers. Such bonding is commonly referred to as silicon-to-silicon bonding and the resulting composite wafer is referred to as an STS wafer. A deep P-N junction can be provided in the STS wafer by selecting as source wafers two wafers having the same crystallographic orientation and opposite conductivity types of desired dopant concentrations as is familiar to those skilled in the art.
Despite the fact that STS bonding is commonly referred to as involving bonding of "bare" silicon, native oxides exist between the wafers. Specifically, oxides grow on the wafer when the wafer is exposed to air.
For conventional bonding of wafers, plasmas are used to remove the impure oxides on the surfaces to be bonded. After such oxide removal, the plasma leaves a thin polymer layer on the wafers. This polymer layer is conventionally thought of as undesirable, so many techniques are used to remove the polymer. Such techniques used to remove polymers include using a mixture of sulfuric acid and peroxide or ashing. After the polymer layer is removed, pure oxide layers are intentionally formed on the wafers. The oxide is formed because silicon will not bond well to silicon. The oxygen thus acts as the bonding medium between the wafers.
However, for STS and other devices this oxidation layer formed between bonded wafers is detrimental to operation. For example, the oxide layer formed at the P-N junction is highly resistive. For power devices this high resistance results in increased power loss across the junction and lower device efficiency.
Therefore, it would be desirable to form bonded wafers without an oxidation layer being formed at the junction of the wafers. It would be desirable if such a device could be formed without the need to shield the wafers from air to avoid formation of native oxides. Such a device would have lower resistance at the bond junction.